Q-switched, continuously-pumped neodymium-YAG lasers typically have nominal pulse widths ranging from 50 to 400 nanoseconds, depending upon the type of laser and the rate of repetition of the pulses. It is often desirable to control the pulse width in order to control the energy carried by a single pulse. For example, when a laser is used to trim thin films on a semiconductor substrate, it is desirable to have short pulses to limit the substrate heating, which could cause undesired changes in physical and electrical characteristics of the substrate.
Laser pulse-width reducers of the type that pass a beam from a pulsed laser through a crystal that rotates the plane of polarization of the beam when a voltage is applied to the crystal have been used in the past. The beam pulse from the crystal, part of it having its initial polarization and part of it having the polarization rotated, is then passed through a polarizer which deflects the portion of the pulse with the rotated polarization into an output beam. With a lithium niobate crystal, 1,800 volts are necessary to rotate the plane of polarization 90.degree. so that the rotated portion will be deflected by the polarizer. By using two crystals in series and having the beam pass through both of them, the voltage necessary to rotate the beam 90.degree. can be reduced to 900 volts. Avalanche transistors or switching tubes operating at up to 4 or 5 kilovolts have been used in the past to drive the crystals. The avalanche transistor systems generally are not reliable and have short lives, and any fast, high-voltage switching causes significant radio frequency emissions, which must be shielded when trimming films on a semiconductor substrate. This is because otherwise the radio frequency emission will interfere with the circuitry receiving small signals from the workpiece to monitor the vaporization of portions of the thin film.